There are various types of filters with different filtering capacities. Examples of well-known filters include those based on cellulose esters, polytetrafluroethylene or polyvinylidene difluoride. They have pores with relatively large average diameters that can range between several hundred and several thousand angstroms with maximum working temperatures of the order of 100.degree. C. to 200.degree. C.
It is also well-known to use filters made from clays or zeolites whose pores can be extremely fine, of the order of some .ANG., which have very diverse applications notably in chemistry, and which are used for example to select chainlike molecules. However, their cost is relatively high.
U.S. Pat. Nos. 2,256,145 or 4,458,528 notably describe the use of porous rock samples for filtering fluids.